Dynamics of the Water Molecules at the Intrinsic Liquid Surface As Seen from Molecular Dynamics Simulation and Identification of Truly Interfacial Molecules Analysis

Abstract

Dynamic properties at the liquid–vapor interface of water are investigated at 298 K on the basis of molecular dynamics simulations and intrinsic surface analysis. The mean surface residence time and diffusion coefficient of the molecules as well as H-bond lifetimes are calculated at the liquid surface and compared to the bulk values. It is found that surface molecules have a non-negligible diffusion component along the surface normal, although this component is limited in time to 7–15 ps, a value comparable with the mean surface residence time. It is also seen that interfacial molecules move considerably faster, and their H-bonds live shorter, than in the bulk liquid phase. This finding is explained by the relation between the number of H-bonded neighbors and mobility, namely that molecules being tethered by more H-bonds move slower, and their H-bonds live longer than in the case of molecules of less extensive H-bonding. Finally, it is found that molecules residing long at the surface are clustering around each other, forming more and longer living H-bonds within the surface layer, but much less outside this layer than other interfacial molecules, indicating that longer surface residence is related to weaker interaction with the subsurface region.